J. Phys. Chem. 1987, 91, 1259-1262
1259
a local order in the head groups in each layer and the appropriate boundary conditions in the relaxation treatment increase the nuclear spin-lattice relaxation rate by a factor of the order of 2.
3. Conclusion
Figure 4. Calculated variations of the spin-label-enhancedlongitudinal nuclear relaxation rate with the distance z between the two parallel planes where the paramagnetic spin-label and the nucleus diffuse in the uniform ( g ( p ) = 1) and nonuniform ( g ( p ) # 1) cases. Here U = 67r(7yysh)*S(S + l)n&,s4 X s, and the other parameters are similar to 7,= those in the legend of Figure 3.
distance z (Figure 1) on the intermolecular nuclear relaxation rate. W e present in Figure 4 the calculated variations of (Tll)-l with z (in A) in the two preceding cases of uniform and nonuniform g(p). One notes a significant increase of this rate in the presence of g(p) for low z values. This is due to the enhancement of the modes of relaxation coming from the numerous reencounters in two-dimensional systems, a situation where the dipolar interactions and the radial molecular distributions are both a t a maximum. For higher values of z the diminution of the dipolar interactions prevails over the local distribution effect. These curves represent some theoretical diagrams for applying the experimental method of spin-labeled lipids or proteins that enhance the nuclear relaxation in other lipids.’-” An application of these curves in the case of metallic paramagnetic compounds for measuring the conformations and lateral diffusion of surfactants in lamellar phases will be given in a forthcoming paper.24 Here again the presence of
W e have used a statistical method to calculate the spectral densities and the spin-relaxation rates associated with a dipolar relaxation mechanism between paramagnetic probes and nuclei, both diffusing laterally in separate parallel planes. The main point of these calculations is the explicit consideration in the molecular dynamics of the intermolecular potential between the head groups in each layer. This has been treated with a Smoluchowski equation with appropriate boundary conditions, especially a t the intermolecular core. The use of finite difference techniques has proven useful to solve this equation numerically and to give convergent expressions of the spectral densities for unbounded lamellar systems, providing we take into account the relaxation of the paramagnetic spin itself. One notes a particularly significant increase of the spin-label-enhanced longitudinal nuclear relaxation rate in comparison with previous treatments. Finally, we present calculated variations of this rate with the diffusion coefficient and the interplanar distance in the two cases of uniform and nonuniform radial molecular distributions. This method could be adapted to 31Pand I3C N M R measurements of diffusion coefficients and conformational properties of molecular surfactants in lamellar phase^.^^^^^
Acknowledgment. J.-P.K. thanks Dr. C. Chachaty (C.E.N. de Saclay, France) for very exciting discussions concerning the spin relaxation in lamellar systems, and Pr. J. H. Freed (Cornel1 University) and Dr. G. Zientara (MIT) for helpful advice about fast computing diagonalizing algorithms for big matrices. Travel funds necessary for completion of this work were given by the “Ecole Polytechnique” (Palaiseau, France) and the “MinistBre Franqais des Relations ExtCrieures”. Computer calculations were made a t the Circe, Orsay, France. (24) Chachaty, C.; Korb, J.-P.; Nery, H., to be published. (25) Chachaty, C.; Quagebeur, J.-P.; Caniparoli, J.-P.; Korb, J.-P.J. Phys. Chem. 1986, 90, 1115.
Silicon-29 Chemical Shifts in Sodalite Materials J. M. Newsam Exxon Research and Engineering Company, Annandale. New Jersey 08801 (Received: August 5, 1986; In Final Form: October 9, 1986)
The 29SiNMR spectra of sodium, lithiuhl, and potassium chloride sodalites are described. The 29Sichemical shift in lithium chloride sodalite, 8 = -76.4 vs. Me,Si, is the lowest yet reported for a Si-4AI environment in a zeolite. When combined with published structural results this datum enables an improved assessment of the functions that have been proposed to describe quantitatively the variation of 8 with local framework geometry. The five functions considered are found to describe the variation of 8(Si-4AI) almost equally well. The 29SiNMR spectrum of the potassium chloride sodalite consists of at least a doublet with peaks at -86.7 and -94.1 ppm, implying that its structure is more complicated than considered previously.
Introduction The peak intensities in 29SiN M R spectra contain information on aluminum distributions and framework compositions. The posThe recent application of 29Si magic-angle spinning nuclear magnetic resonance (NMR) to zeolites’J has been most f r ~ i t f u l . ~ - ~ itions of the peaks, that is the 29Sichemical shifts 8, depend on the number of aluminum atoms in the first’J and second6 coordination shells and they are also sensitive to changes in the local (1) Lippmaa, E.; Magi, M.; Samoson, A.; Engelhardt, G.;Grimmer, A. framework geometry.”” For a wide range of silicates and R. J . Am. Chem. SOC.1980, 102, 4889-4893. (2) Lippmaa, E.; MBgi, M.; Samoson, A.; Tarmak, M.; Engelhardt, G.J . ~~
Am. Chem. SOC.1981, 103, 4992-4996. (3) Fyfe, C. A,; Thomas, J. M.; Klinowski, J.; Gobbi, G . C. Angew. Chem., Int. Ed. Engl. 1983, 22, 259-336.
0022-3654/87/209 1- 1259$0 1.50/0
(4) Klinowski, J. Prog. NMR Spectrosc. 1984, 16, 237-309. (5) Oldfield, E.; Kirkpatrick, R. J. Science 1985, 227, 1537-1544. (6) Newsam, J. M. J . Phys. Chem. 1985, 89,2002-2005.
0 1987 American Chemical Society
1260 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
aluminosilicates, general correlations have been demonstrated between 6 and bond strength sums14v5and group electronegativity sums.18 These general correlations cover a wide range of chemical shifts, but they cannot reproduce, for example, the observed variations in d(Si-4Al). Correlations suitable for describing observed chemical shift variations in more limited classes of materials such as the Q4 silicates and zeolites (Q4 aluminosilicates) have also been widely explored. Linear relationships have been proposed to describe the variation of G(Si-OA1) with the mean S i U T bond angle, 8" ( T tetrahedral species, Si or Al), G(Si-OA1) with the mean of the secants of the Si-0-T bond angles,' G(Si-nAl; n = 0-4) with Cdrr (where drr is the separation between the target 29Sinucleus and its first neighbor T atom, drr being computed from the relevant T-0-T angle),I3 and S(Si-OA1) with cos $/(cos 8 - 1).15,16 In these earlier s t ~ d i e s ~ ,the ~ ~emphasis J ~ ~ ' ~ was on the Si-OAl chemical shifts which, unfortunately, are the most sensitive to framework composition/second-neighbor effects6 The Si-4A1 peak positions are less susceptible to such effects because, by Loewenstein's rule,I9 no aluminum atoms can occupy secondneighbor sites. The range of chemical shifts observed to date for Si-4AI environments is, however, rather limited, preventing a detailed analysis of the variation of S(Si-4Al) with local framework geometry. In the present report we describe 29SiN M R studies of ion-exchanged sodalites and a reassessment of the various functions proposed to describe the variation of d(Si-nAl) with local framework geometry.
la Newsam
Li
il
J
-55
Experimental Section Sodium chloride sodalite, Na6Si&&'1.6NaCI, was prepared by procedures similar to those described by Barrer et aLzo Lithium- and potassium-exchanged materials were prepared by molten salt exchange of this sodium chloride sodalite using the procedures described by Taylor.21 Least-squares optimization of the cubic lattice constants using reflections in the range 10 < 20 < 80" gave values of 8.897 (1) (Na), 8.461 (1) (Li), and 9.235 (1) A (K) which agree reasonably with respective literature values of 8.879 (Na), 8.447 (Li), and 9.253 A (K).22 The powder diffraction X-ray profile of the potassium-exchanged material showed two medium-weak lines a t d = 3.1 17 A and d = 2.607 A that could not be indexed on the basis of this cubic unit cell. Analysis of the parent sodium chloride sodalite by a combination of chemical assay (for CI) and inductively coupIed plasma emission spectroscopy (ICPES) gave ratios of Si:Al = 0.97, Na:AI = 1.28, and C1:Na = 0.21, consistent with the above formula. ICPES analyses of the exchanged materials gave relative cation c o m p sitions for the Li- and K-exchanged sodalites of Lio,99Nao,ol and Ko,8sNao,12, with corresponding Si:Al ratios of 0.98 and 1.01, respectively. 29SiN M R spectra were recorded on a JEOL FX200WB spectrometer operating at 4.7 T using magic-angle sample spinning (7) Thomas, J . M.; Fyfe, C. A.; Ramdas, S.; Klinowski, J.; Gobbi, G. C. J . Phys. Chem. 1982, 86, 3061-3064. (8) Smith, J. V.; Blackwell, C. S . Nature (London) 1983, 303, 223-225. (9) Jarman, R. H. J . Chem. SOC.,Chem. Commun. 1983, 512-513. (10) Jarman, R. H.; Melchior, M. T.; Vaughan, D. E. W. In Intrazeolite Chemistry, Stucky, G. D., Dwyer, F. G., Eds.; American Chemical Society: Washington, DC, ACS Symp. Ser. No. 218, 1983; pp 267-281. (1 1) Thomas, J. M.; Klinowski, J.; Ramdas, S.;Hunter, B. K.; Tennakoon, D. T. B. Chem. Phys. Lett. 1983, 102, 158-162. (12) Fyfe, C. A,; Gobbi, G. C.; Murphy, W. J.; Ozubko, R. S.; Slack, D. A. Chem. Lett. 1983, 1547-1550. (13) Ramdas, S.; Klinowski, J. Nature (London) 1984, 308, 521-523. (14) Magi, M.; Lippmaa, E.; Samoson, A,; Engelhardt, G.; Grimmer, A. R. J . Phys. Chem. 1984,88, 1518-1522. (15) Engelhardt, G . ;Radeglia, R. Chem. Phys. Lett. 1984, 108, 271-274. (16) Radeglia, R.; Engelhardt, G. Chem. Phys. Lett. 1986, 114, 28-30. (1 7) Newsam, J. M.; Jarman, R. H.; Jacobson, A. J. J . Solid State Chem. 1985, 58, 325-334. (18) Janes, N.; Oldfield, E. J . Am. Chem. SOC.1985, 107, 6769-6775. (19) Loewenstein, W. Am. Mineral. 1954, 39, 92-96. (20) Barrer, R. M.; Cole, J. F.; Sticher, H. J . Chem. SOC.A 1968, 2475-2485. (21) Taylor, D. Contrib. Mineral. Petrol. 1975, 51, 39-41. (22) Beagley, B.; Henderson, C. M. B.; Taylor, D . Mineral. Mag. 1982, 46, 459-464.
.
I
-65
,
I
-75
'
I
-85
1
1
-95
,
I
-105
Chemlcal ShlR (ppm vs TMS)
Figure 1. 29SiN M R spectra of lithium (a), sodium ( b and c), and potassium (d) chloride sodalites recorded by magic-angle sample spinning and with DSS as an internal standard (see text). The two sodium sodalite spectra are respectively that of the parent material (b) and that derived by back-exchange of the potassium exchanged sample (c). The chemical shift scale is drawn referenced to Me4Si.
and with approximately 20% sodium 4,4-dimethyl-4-silapentanesulfonate (DSS) as an internal standard. The sodium and lithium sodalites gave single, sharp peaks centered at -85.3 (with respect to tetramethylsilane (Me4.%)) and -76.4 ppm, respectively (Figure 1). All of the spectra also contain a very broad component presumed to derive from a second, amorphous phase. This broad component is commonly observed in zeolite samples and, for the materials discussed here, it varied little from one sample to another. The potassium sodalite, expected on the basis of the T-0-T angle of 155.4' observed in the powder X-ray diffraction study22 and the analysis presented here (Table 11) to give a single peak a t ca. -92.0 ppm, gave two peaks a t -86.7 and -94.1 ppm, respectively (Figure 1). The centroid of the pattern at -90.2 ppm is reasonably close to the expectation value. The two peaks in the spectrum cannot be assigned respectively to Si-4Al and Si-3Al components that might have been observed had significant framework dealumination accompanied the ion-exchange procedure. The separation between the peaks of 7.4 ppm is larger than that expected to separate Si-4Al and Si-3A1 components, and their approximately equal intensities would be difficult to rationalize in terms of reasonable Si-A1 distributions in the structure. Further, although dealumination of the sodalite framework has not yet been explored, the compactness of the structure and the presence of chloride ions within the sodalite cages must make accomodation of detrital, nonframework aluminum species difficult. The chemical analysis of the potassium chloride sodalite (above) is therefore also evidence against the occurrence of dealumination in these materials. As a final test, the potassium sodalite sample was back-exchanged by twice-repeated treatment in molten NaCl at 830 O C for 2 h. The product analysis by ICPES gave an Si:A1 of 0.97 and an Na:Al of 1.33. The powder X-ray diffraction pattern was similar to the parent sodium chloride sodalite with a cubic unit cell of 8.886 (1) A. The 29SiNMR spectrum, likewise, is similar to that of the parent material and gives an almost identical chemical shift of -85.1 ppm (Figure 1). The data thus suggest that the published structural model for potassium chloride sodaliteZ2is only approximate, and that it
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1261
29Si Chemical Shifts in Sodalite Materials
TABLE I: Si-4Al Chemical Shift and Structural Data for Aluminosilicate Zeolites
framework Si/AI cation ABW 1.0 Li LTA FAU CAN
SOD THO
1.0 1.0 1.18 2.31 2.37 1.0 1.0 1.0 1.0 1.0
Na Li Na Nab Na Na Nab Na Li Ca/Na
Si-4A1 6 ref -80.1' 27 -81.7 29 -88.9 2,27 -85.0 31 -84.6' -84.1d -84.1d -87.2 27 -86.7 27 -85.3 41 -76.4 41 -83.5 2
01 139.1 139.1 145.5 174.2 132.4 141.7 133.0 148.6 138.7 138.2 125.6 139.2 140.9 138.0
02 136.9 136.9 159.5 130.8 141.5 144.5 143.0 150.0 138.7 138.2 125.6 139.2 127.7 134.1
03 140.5 140.5 144.1 129.9 136.3 141.3 136.0 135.4 138.7 138.2 125.6 137.2 140.3 134.0
04 123.1 123.1 144.1 129.9 146.8 141.4 133.0 135.4 138.7 138.2 125.6 137.2 126.4 136.7
-
0 sec (TOT) 134.9 -1.4167 134.9 -1.4167 148.3 -1.1753 141.2 -1.2831 139.3 -1.3200 142.2 -1.2651 136.3 -1.3843 142.4 -1.2630 138.7 -1.3311 138.2 -1.3414 125.6 -1.7179
sec -1.4549 -1.4549 -1.1875 -1.4134 -1.3348 -1.2659 -1.3937 -1.2838 -1.3311 -1.3414 -1.7179
Edrr 12.449 12.449 12.967 12.715 12.637 12.754 12.509 12.759 12.614 12.593 11.989
135.9
-1.4061
12.494 0.4180 0.4166
-1.3925
p
0.4138 0.4138 0.4597 0.4380 0.4310 0.4415 0.4194 0.4419 0.4290 0.4271 0.3679
p 0.4103 0.4103 0.4576 0.4189 0.4292 0.4414 0.4183 0.4391 0.4290 0.4271 0.3679
ref 28 28 30 29' 35 37 38 39 40 42 22 43
'Datum neither plotted nor used in the regression analyses. bData plotted in Figures 2 but not used in the regression analyses. CAverageof Si-4AI chemical shifts for samples 1 and 2 in ref 32, 1 and 2 in ref 33, and 2 and 4 in ref 34 (see ref 6). dAverage of Si-4A1 chemical shifts for sample 6 in ref 32, 4 and 6 in ref 33, and 1 in ref 36 (see ref 6). CStructuraldata from refinements in space group F d c . TABLE 11: Regression Analyses Results
function," x mean T-0-T, 8 mean sec (T-0-T) ,mTT
P
P
gradient, M intercept, C -0.555 (51) -7.462 (7.040) -23.83 (1.80) -117.3 (2.5) 77.99 (12.42) -12.89 (99) -25.97 (4.30) -137.0 (10.1) -143.27 (10.0) -23.82 (4.23)
'The computed chemical shift, 6 = M x relation coefficient.
+ C.
Rb -0.972 -0.981 -0.980 -0.981 -0.983
b R is the linear cor-
actually contains a t least two inequivalent silicon sites. This structural ambiguity unfortunately prevents the use of the data for the potassium-exchanged material in considering quantitatively the structural dependence of the 29Sichemical shifts. It is noteworthy, however, that the second peak and the centroid occur a t chemical shifts significantly larger than the 6(Si-4A1) chemical shift of -88.9 ppm in zeolite A which, a t one stage, was regarded as a n o m a l o ~ s . ~ The ~ , ~ structural ~ results of Beagley et a1.22 demonstrate that the flexibility in the condensed SOD is sufficient for a 30° span of T-O-T angles to be accessible by appropriate ion exchange. The present 29SiN M R results for these materials thus apparently extend the range of chemical shifts observed for the Si-4AI environment in zeolites a t both high- and low-field ends. The available structural and 29Sichemical shift data for Si-4Al environments in zeolites are collated in Table I. Where necessary (23) Engelhardt, G.; Zeigan, D.; Lippmaa, E.; Magi, M. Z . Anorg. Allg.
Chem. 1980. 468. 35-38.
(24)Buriill, L: A,; Lodge, E. A,; Thomas, J. M.; Cheetham, A. K. J. Phys.
Chem. 1981, 85, 2409-2421.
(25) Dempsey, M. J.; Taylor, D. Phys. Chem. Miner. 1980,6, 197-208. (26) DeDmeier. W. Acta Crystalloar.. Sect. B 1984. 40. 185-191. (27) Klkowski, J.; Thomas,-J. M.cFyfe, C. A.; Hartman, J. S. J. Phys. Chem. 1981.85, 2590-2594. (28) Kerr, I. S. Z . Kristallogr. 1974, 39 186-195. (29) Newsam, J. M., manuscript in preparation. (30) Gramlich, V.; Meier, W. M. Z . Kristallogr. 1971, 133, 134-149 (31) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J.; Pictroski, C. F. In Proceedings of the Sixth International Zeolite Conference Olson, D. H., Bisio, A,, Ed.; Butterworths: Surrey, UK, 1984, pp 684-693. (32) Klinowski, J.; Ramdas, S . ; Thomas, J. M.; Fyfe, C. A.; Hartman, J. S.J. Chem. SOC.,Faraday Trans. 2 1982, 78, 1025-1050. (33) Engelhardt, G.; Lohse, U.; Lippmaa, E.; Tarmak, M.; Magi, M. Z . Anorg. Allg. Chem. 1981, 482, 49-64. (34) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J. J . Am. Chem. SOC.1982, 104, 4859-4864. (35) Olson, D. H. J. Phys. Chem. 1970, 74, 2758-2764. (36) Engelhardt, G.; Lohse, U.; Samoson, A.; Magi, M.; Tarmak, M.; Lippmaa, E. Zeolites 1982, 2, 59-62. (37) Hseu, T. Ph.D. Thesis, University of Washington, 1972. University Microfilms, No. 73-13835, Ann Arbor, MI. (38) Mortier, W. J.; Van den Bossche, E.; Uytterhoeven, J. B. Zeolites 1984.4. 41-44. (39) ~BrescianiPahor, N.; Calligaris, M.; Nardin, G.; Randaccio, L. Acta Crystallogr. Sect. B 1972, 338, 893-895. ~
the T-0-T angles have been computed from the published atomic coordinates. These data, except as discussed below and noted in Table I, were used to compute the least-squares best lines for each of the five proposed linear relationships introduced above. The results are given in Table I1 and plotted in Figure 2. The ABW-framework chemical shift was taken from the more recent measurements using an internal standard.2 Similarly, the basic sodalite datum from ref 27 is plotted in Figure 2 but was not used in the regression analysis. For sodium zeolite Y, two somewhat differing structural models are a ~ a i l a b l e .Although ~ ~ ~ ~ ~ the X-ray diffraction study of a sodium-exchanged natural faujasite crystaP7 is the more precise study, the powder X-ray diffraction results of Mortier et aL3*are considered to model more accurately the materials used in the 29Si N M R measurements. The latter structural data were therefore used for the regression analysis, although the data from both sources are plotted in Figure 2.
Discussion and Conclusion The 29Sichemical shift in lithium sodalite is considerably smaller than those observed previously for Si-4Al environments in zeolites. It is, however, consistent with that expected on the basis of the known structural data22and the published relationships between 6 and the Si-0-AI angles.8~11~13~15~16 The data used in the regression analyses (Figure 2 and Table I) include points for both sodiumand lithium-exchanged zeolites. The centroid of the potassium sodalite spectrum is also generally consistent with these trends. These results thus demonstrate that the direct effect of these differing nonframework cations is small. Their larger influence on the 29Si chemical shifts derives from the adjustment of the Si-0-T angles that reflects their differing coordination requirement~.~~ The five functions of the Si-0-T angle, 0, describe the variation of 6(Si-4A1) almost equally well (Table 11). The linear correlation coefficients, RL,are high and similar. Bonding considerations indicate that the fractional s character of the oxygen hybrid orbitals as a function of the bond angle, 0, is best described by function p44and the functions p and p do give marginally higher RL values consistent with this a r g ~ m e n t . ] ~ The . ' ~ similar performances of the other functions presumably indicate that, over the range of angles involved, they quite accurately approximate the variation of p. The general relationship between 6(Si--OAl) and the Si-0-T angles has already been exploited in assigning the 29SiN M R spectra of, for example, synthetic m a ~ z i t eand ~ ~ certain cla(40) Hassan, I.; Grundy, H. D. Acfa Crystallogr., Sect. C 1983, 39, 3-5. (41) This work. (42) Hassan, I.; Grundy, H. D. Acta Crystallogr., Sect. B 1984, 40, 6-13. (43) Pluth, J. J.; Smith, J. V.; Kvick, A. Zeolites 1985, 5 , 74-80. (44) Coulson, C. A.; Valence; Oxford University Press: London, 1961. (45) Jarman, R. H.; Jacobson, A. J.; Melchior, M. T. J . Phys. Chem. 1984, 88, 5748-5752.
1262 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Newsam -70.0
-70.0 ,
~
-82.0 -86.0 -
I
120.0
126.0
132.0
138.0
Mean T-0-T
144.0
150.0
0.350
0.380
Angle (")
0.410 p = cos3
0.440
/(case
0.470
0.500
- 1)
-70.0
fI-
-74.0,
-EEo P
c
v)
1
-90.0 -1.800
1
I
1
-1.640
-1.480
-1.320
-1.160
-1.000
-90.0 0.350
I
0380
I
0.410
I
0.440
I
0.470
0.500
Mean Secant (T-0-T)
S -74.0 I-
-78.0
I
6 -86.0 I
-90.0 11.500
1
11.800
1
1
12.100 WT
12.400
12.700
13.000
(4
Figure 2. 29Sichemical shifts, 6(Si-4A1), referenced to Me& plotted as various functions of the Si-0-A1 angles, 0. The functions used as ordinates in the five plots are discussed in the text. The parameters describing the least-squares best lines through the data are listed in Table 11. The two data points that were not used in the regression analyses are indicated by the modified symbols (see text).
thrasils.& Although the number of zeolites that have Si:Al ratios sufficiently low for the Si-4AI peak in the 29SiN M R spectrum to be well defined is limited, if the Si-0-T angles are known for each of the crystallographically unique silicon atom sites, 6(Si-4A1) can be calculated (Table 11). This datum can then, in principle, be combined with a knowledge of the quantitative effects of first and second neighbor aluminum atoms and the framework topology to compute the chemical shifts, b(Si-nAl); n = 0-4, at other (46) Groenen, E. J. J.; Alma, N. C. M.; Dorrepaal, J.; Hays, G. R.; Kortbeek, A. G. T. G. Zeolites 1985, 5, 361-363.
framework compositions! This methodology may thus enable the complete assignment of the 29SiN M R spectrum of any zeolite whose structure is known. The converse, the extraction of detailed information about unknown structures from their 29Si NMR spectra, remains difficult. The values of the Si-0-T angles are not characteristic of a particular framework topology, as is well illustrated by the sodalite samples studied here.
Acknowledgment. I thank J. Dunn for help in the sample syntheses, H. Malone for accumulating the N M R data, and R. H. Jarman and M. T. Melchior for helpful discussions. Registry No. NaC1, 7647-14-5; LiCI, 7447-41-8; KCI, 7447-40-7.
